Hello FreeBSD users,
I tried FreeBSD and was very impressed to see the audio mixing without problem (to be able to have two applications running at the same time without one of them lock the audio device)

Are you only using OSS ? How can you get a mixed audio with only OSS ?
I am on NetBSD and everytime I run a software that requires audio, it will lock the audio up and no other software will have sound, which is seriously annoying when you are using at least two softwares that need sounds

which is an intermediate layer between audio or MIDI programs and the hardware. OpenBSD just like all BSDs, Solaris, and Linux in the distance past use OSS. If you are musician you might want to look at OpenBSD which is commonly used as a mixer platform for large rock concerts. OpenBSD plays multiple sound streams out of box of course just like OS X or Windows.